The present invention relates to a technique for achieving stable locking using a cross modulation carrier in a laser light generating apparatus with a laser light source and a plurality of resonators.
An FM sideband method (“Pound-Drever-Hall Locking” method) is known as a method for stable locking of a resonator in a laser light generating apparatus, and is widely used for the locking of an external resonator in a range of near-infrared light to visible light.
In addition to this method, there is a method using polarization (see literature such as Hansch, Couillaud, optics Communications, 1980). For example, a locking method has been proposed which inputs laser light passed through a wave plate to a resonator, and obtains an error signal on the basis of detection signals of two beams in different polarized states from the resonator to thereby maintain a resonant state of the resonator.
However, the conventional apparatus have the following problems.
In the case of the polarization method and the like, when the amount of light inputted to the resonator or the like is changed or when an absorptive optical element (especially an element absorbing light in an ultraviolet range) is included in the resonator, for example, a wavelength offset tends to occur because characteristics of the optical element are changed with variation in temperature due to generation of heat and thus the amount of phase delay is changed. The polarization method and the like therefore have a problem in stability.
The FM sideband method or the “Pound-Drever-Hall Locking” method requires that sideband waves be generated using a phase modulator in order to generate an error signal. A high-performance phase modulator having a high transmissivity and a low operating voltage is required especially when ultraviolet light is modulated to thereby generate sideband waves. However, such a modulator is difficult to obtain. When an available phase modulator is to be used, on the other hand, a high-voltage high-frequency driving circuit and the like are required. The FM sideband method has a disadvantage of increasing cost and power consumption and thus has a problem in terms of practicality.
In a configuration using a plurality of resonators, when the resonators are to be locked simultaneously, the increase in the number of resonators results in an increase in the number of phase modulators disposed in different stages, thus causing an increase in the size and cost of the apparatus. This is because in the conventional configuration, a phase modulator is disposed in a stage preceding each of the resonators. A space for disposing the phase modulators thus becomes larger.